From the onset of internal pipping (i.e. embryonal breathing of air cell gas) until hatching, the chorioallantoic membrane (CAM) and the lungs work simultaneously to serve the metabolic needs of the embryo. (1) The carbon dioxide and oxygen exchange rates (V&dot;O2 and V&dot; CO2) through the lungs and the CAM were separately, but simultaneously, measured during the last two days of incubation (day 20--21), while ventilation (V&dot;E) was calculated from the measurements of pressure oscillations in the air cell. When the embryo's total metabolic rate was increased, V&dot;E was linearly proportional to lung V&dot; O2 and V&dot;CO2 and not to the embryo's total metabolic rate. (2) Tracheal pressure and changes in lung volume were quantified through mechanical ventilation of the embryo. The curled up posture of the embryo, the eggshell and its membranes did not represent a significant mechanical constraint to V&dot;E. (3) V&dot; E, lung V&dot;O2 and V&dot;CO 2 were measured while the CAM compartment was exposed to either 10% O2, 100% O2 or 5% CO2. Total V&dot; O2 was also measured under these conditions. There is a clear V&dot; E-sensitivity to CO2 and a rather weak V&dot;E-sensitivity to changes in arterial oxygenation present at this stage of development.